Tyre Pressure Monitor Module

Receives pressure readings from sensors inside the wheels — and it is unusual on the vehicle's network in one respect that explains most of its faults: the sensors are not wired to anything. They transmit by radio, and each is a small sealed unit with a non-replaceable battery lasting roughly five to ten years.

That produces a failure mode nothing else on the vehicle has: sensors that simply expire, usually several within a short period, because they were all fitted at the same time. A car of the right age reporting implausible pressure data is very often reporting dying batteries — an end-of-life matter rather than a fault to diagnose.

Two other causes are procedural and both follow tyre work. Each sensor has an identifier the vehicle associates with a wheel position, so rotating tyres or fitting different wheels without a relearn leaves stale associations — a pressure reported for the wrong corner, or a sensor the car can no longer find. And a tyre machine can break a sensor easily, since it sits on the valve stem inside the wheel, which is why a fault dating from the last tyre change usually is that.

The safety point outranks the diagnosis: an under-inflated tyre overheats, wears out, handles badly and can fail at speed. With the monitoring untrustworthy, pressures need checking by hand with a gauge, regularly.